1. Field of the Invention
The present invention relates to a method for measuring a sensing margin of a sense amplifier for a memory device, and more particularly to, a method for measuring a sensing margin generated by a gate residue in a memory cell transistor.
2. Description of the Related Art
In general, high speed memory devices including DDR, DDR2 and DDR3 SDRAM use a transistor having a low threshold voltage to satisfy speed-related specifications such as tRCD, tRP and tAA.
Especially, the transistor having the low threshold voltage is employed for a sense amplifier directly relating to a read operation and peripheral transistors.
However, when the transistor having the low threshold voltage is used, if a test mode for sensing margin control tm_smc which is a test mode for screening a gate residue of the transistor during the manufacturing process is executed, a breakdown is generated in the transistors composing the sense amplifier before the gate residue is screened. It is thus difficult to normally screen the gate residue.
The test mode for sensing margin control tm_smc will now be explained in detail with reference to FIG. 1.
FIG. 1 is a circuit diagram illustrating bit lines, a sense amplifier and peripheral circuits of a general memory device.
The test mode for sensing margin control tm_smc measures a sharing time of effective charges stored in a capacitor C11 of a cell transistor TR13 turned on by a word line WL and transmitted to a bit line BIT. When sharing of the charges occurs, a voltage of the bit line BIT is VBLP-ΔV and a voltage of an inverted bit line /BIT is VBLP. Here, VBLP denotes a bit line precharge voltage, and ΔV denotes a minute voltage difference generated by charge sharing.
If a bridge is formed between the bit line BIT and a gate (namely, ‘a’ region) due to a gate residue generated on the cell transistor TR13 during the manufacturing process, a high voltage of the word line WL is transmitted to the bit line BTL through the cell transistor TR13, to offset effects of the minute voltage difference ΔV generated by charge sharing. As a result, failure of the cell transistor TR13 can be screened in the test mode for sensing margin control tm_smc.
However, in the test mode for sensing margin control tm_smc, isolated transistors TR11 and TR12 are turned on. Accordingly, the bit lines are coupled to the sense amplifier, so that characteristics of the transistors for the sense amplifier may influence on the screening results of the test mode for sensing margin control tm_smc.
For example, when the transistor having a low breakdown voltage is used for the sense amplifier, the bit line BIT and the inverted bit line /BIT are electrically connected through the transistors composing the sense amplifier. In this case, failure of the cell transistor TR13 cannot be normally screened.